1. Field of the Invention
The present invention relates to an antenna pointing equipment which, for inter-satellite communication between a geostationary satellite and a low earth orbit satellite, is adapted to point an antenna carried on one of the satellites to the other.
2. Description of the Related Art
In general, in transmission of mission data obtained by a low earth orbit satellite to an earth station or control command prepared by the earth station to the low earth orbit satellite, long-time communication is secured via the geostationary satellite. In order to permit communication between the geostationary satellite and the low earth orbit satellite, it is necessary that, in each of the satellites, its antenna be driven to track the other.
Usually, as shown in FIG. 1, a geostationary satellite A is placed in an geostationary orbit at a height of approximately 35,786 kilometers and moves in synchronism with the earth's rotation, while a low earth orbit satellite B such as an observatory satellite moves in a low earth orbit substantially from south to north and vice versa. The antenna pointing mechanism of an intersatellite communication antenna B1 carried on the low earth orbit satellite B comprises an azimuth (Az) axis driving unit B2 and an elevation axis (E1) driving unit B3. The Az axis driving unit B2 rotates with its rotation axis pointed in the direction of the earth's center, while the E1 axis driving unit B3 rotates with its rotation axis parallel to the horizontal direction on the earth's surface. The antenna B1 is fixed to the E1 axis driving unit B3 and its direction is controlled by amounts of rotation of the units B1 and B2.
In implementation of intersatellite communication, when the low earth orbit satellite B comes into the field of view of the geostationary satellite A, the null axis of the antenna B1 is roughly directed to the geostationary satellite A in an acquisition control mode to acquire radio frequency beacon (signals or light) from the geostationary satellite. At the completion of the acquisition of radio frequency beacon, data communication is initiated and the operation is switched to a tracking control mode in which the antenna driving unit B3 is driven to track the geostationary satellite A until the strength of received signals or beacon is maximized.
The orbit of the low earth orbit satellite B varies from hour to hour as the earth rotates and thus, as shown in FIG. 2, the geostationary satellite A may pass through the neighborhood of the zenith (the point on the extension of the Az axis, which is referred to as the singular point) as seen from the low earth orbit satellite B. In such driving case, the Az driving unit B2 must be rotated at high speed in order to track the geostationary satellite A. However, in order to realize the Az driving unit B2 to such high-speed rotation, a large motor must be used, thus making the unit large and increasing power dissipation. This is not desirable for equipment which is to be carried on satellites.
From the above, with the conventional antenna pointing equipment carried on satellites, the portion indicated by oblique lines in FIG. 3 is regarded as an area impossible to track and, as soon as the geostationary satellite enters that area, the mode of operation is changed from the tracking control mode to the acquisition control mode, thereby acquiring radio frequency beacon again after the passage through the area. Under the present conditions, however, it takes a very long time to acquire the radio frequency beacon again. Communication becomes impossible while the radio frequency beacon is being acquired. Therefore, it is strongly desired that the accuracy of acquisition be improved and a period of time during which communication is impossible be shortened.